America Plate Research Articles

Exhumation response to oceanic plateau accretion and oroclinal bending: Low-temperature thermochronology study of Wrangellia terrane on southern Vancouver Island, Canada

Approximately 50 Myr ago, the triple junction of the Kula-Farallon-North America plates converged with the continental margin, causing ridge subduction and the formation, accretion and translation of two oceanic plateaus. We investigate the effects of this tectonic configuration on the exhumation of southern Wrangellia terrane on southern Vancouver Island since the Eocene. We report late Cretaceous to late Oligocene (85.4 to 23.3 Ma) apatite fission track ages (AFT) and, for the first time, Oligocene to early Miocene (36.6 to 14.0 Ma) apatite (UTh)/He ages (AHe) for 16 bedrock samples of Wrangellia. The thermal history modelling of these ages for 13 samples reveals variable cooling patterns between regions. Samples close to the major faults of a fold and thrust belt show accelerated cooling (4–5 °C/Myr) during the Eocene. In the central area, the modelled cooling rates have been slow and generally uniform throughout the Cenozoic (0.5–1.5 °C/Myr), whereas samples from the west coast yielded very slow cooling (<0.5 °C/Myr) from 70 to 30 Ma, followed by moderate cooling (1.5–3 °C/Myr) since. Combining ages, fission track length and thermal history models in this and previous studies, we interpret the moderate-accelerated exhumation of the fold and thrust belt in the Eocene to be a response to oroclinal bending following oceanic plateau accretion. The exhumation pattern of the western side of southern Wrangellia is linked to the ongoing Cascadian Subduction zone ca.30 Ma. This exhumation pattern also supports a hypothesis that all crust of southern Wrangellia was all overlain by sedimentary strata in Eocene before ∼50 Ma, and that an accretionary complex of the Pacific Rim terrane was partly the outboard equivalent of these strata. In the southern Wrangellia, no exhumation response to the Miocene oroclinal bending associated with formation of the Olympic mountains is observed.

Crustal Structure of Southern California from Velocity and Attenuation Tomography

Recent studies of Southern California have employed velocity and attenuation tomography to elucidate two aspects of crustal structure. Low-frequency velocity tomography reveals large-scale heterogeneity that can be approximated by a discrete set (𝐾 ≤ 10) of tectonic regions, each characterized by an isostatically balanced lithospheric column reflecting the composition and tectonic history of the region. The boundaries between tectonic regions are typically localized structures expressed at the surface by major faults, topographic fronts, and geochemical transitions. The efficacy with which tomographic regionalization (TR) represents the subsurface geography of major tectonic units in Southern California indicates that the TR idealization works surprisingly well in this part of the Pacific‐North America plate boundary. High‐frequency attenuation tomography in Southern California supports the hypothesis that the decay of P and S waves propagating through the upper and middle crust is dominated by elastic scattering from small-scale heterogeneities of high fractal dimension, rather than by anelastic dissipation. The amplitude of the scattering varies systematically among the tectonic regions and correlates with the degree of recent faulting and deformation within a region. These results motivate speculation that small-scale crustal heterogeneities responsible for high-frequency attenuation are generated within the seismogenic zone through a dynamic interplay between distributed deformation and localized fracturing.

Santos Basin as an Example of a Shear-Driven Core Complex in a Transform Marginal Plateau

It is proposed a new model on the kinematics of strain partitioning for the breakup of the Santos and Namibe conjugate basins. The model is based on observed strike-slip corridors and low-angle detachments with metamorphic core complexes, which accommodated the deformation in a mega relay zone. Through detailed reconstructions of this segment of the South Atlantic, collecting multidisciplinary data from previous researchers, and considering the relative relationship between Africa and South America during the breakup process, it is herein reexamined key evidence of how the Brazilian side became so wide, and acted as an accommodation zone during the Aptian. The 600 km Long transpressional-dextral Proterozoic Ribeira Belt is the cradle of the transtensional-sinistral Santos-Namibe strike-slip rift propagator. These conjugate basins evolved as a large-scale relay zone, developing an oblique-left-lateral extensional corridor during the Aptian, balancing mechanically coeval deformations from two sub-parallel spreading branches, traveling from North and South, but hundreds of kilometers apart from each other. Proterozoic inheritance, and the dynamic clockwise rotation of South America (far-field stresses) controlled strain partitioning between the Campos/Benguela basins and the Pelotas/Walvis basins. The onset of seafloor spreading around the Falkland (Malvinas) Island, triggered the relative clockwise rotation of the southernmost tip of the South America plate, and the intrusion of transversal dike swarms of the Ponta Grossa Arch, which is interpreted as fissural magmatism, comparable to a regional opening mode(type I) fracture system. Plate kinematic transport direction inferred from plate reconstructions is mainly EW. The step-over oblique-slip of this relay zone widened the Santos Basin in the NW-SE direction, often mistakenly misinterpreted as the extension direction in the Santos Basin. This NW path is the direction of the maximum elongation within an E-W shear corridor. Our work details a new and unprecedented strike-slip structural framework of the Santos Basin, with large-scale basement-involved folding around the Outer High, which evolved to an obliquely sheared active low-angle detachment system, simultaneously influenced by the thermal anomaly of the Tristan-Gough plume, responsible for magmatic thermal weakening and thermal-induced uplift. Magmatic underplating may have facilitated the doming process. The Santos Basin is located at the heart of a Transform Marginal Plateau, as previously proposed. It is composed of a magmatic crust, with fragmented slices of continental crust, obliquely sheared during successive transform movements, with possible magmatic underplating. This kinematically linked system of normal faults, strike-slip faults, flexural folds, and detachment faults has direct impact on understanding the properties of world-class oil and gas reservoirs in the Brazilian Pre-Salt Supergiant Province. With the ongoing regional transgression, the active structural highs were the site of deposition of the carbonate reservoirs that host an in-place volume around a hundred billion barrels of hydrocarbons, considering the whole province.

Controls on the genesis of a giant sand injection complex; insights into the Paleogene evolution of the stress of northern and central California

Giant sand injection complexes and localized swarms of sandstone intrusions are common in Upper Cretaceous to Miocene sedimentary successions of central and northern California within a distance of less than 100 km from the Pacific margin of the North America plate. One of the best preserved and extensively exposed injection complexes is the late Eocene Tumey Giant Injection Complex. The emplacement of sand injectites was driven by overpressure generated by thermal diagenesis of biosiliceous and smectite-rich mudstone host-rocks. The orientation and size distribution of sandstone intrusions was controlled by stress in which σ 1 and σ 3 were horizontal and, respectively, parallel and perpendicular to the present trace of the San Andreas Fault, and σ 2 was vertical. A strike-slip tectonic regime is inferred. Our analysis documents margin-orthogonal extension and provides support for a late Eocene phase of increase of strain, and possibly active slip, along a syn-subduction strike-slip fault zone. Comparison with other injection complexes in the region indicates that the near-field maximum principal stress rotated through time, from normal to parallel with respect to the plate margin, probably in relation to variations of the relative motion vector of the converging plates.

Joint flexural-density modeling of the Taltal, Copiapó, and Iquique hotspot ridges and the surrounding oceanic plate, offshore Chile

Abstract Based on gravity and bathymetric data and using a novel two-dimensional joint flexural-density modeling approach, this work studies the physical properties of the oceanic Nazca plate around the Taltal, Copiapó, and Iquique hotspot ridges offshore northern Chile. The area is located westward of the Chilean Trench where the Taltal and Copiapó Ridges collide with the continental margin. The results show that the variability in density structure at different scales is a key factor in explaining the observed gravity signal, playing an important role in the lithospheric flexure and hence the elastic properties of the Nazca plate in this setting. The results can be interpreted as evidence of spatial and temporal heterogeneities in the plate-weakening process at the hotspots, magmatic underplating, and crustal and upper mantle fracturing and/or hydration. These processes might be relevant for the ascent of magma pathways of later (secondary) volcanism and influence the mechanical segmentation of the oceanic plate. The latter is critical in explaining the active seismogenic contact between the oceanic Nazca and overriding South America plates.

Formation and Evolution of the Pacific‐North American (San Andreas) Plate Boundary: Constraints From the Crustal Architecture of Northern California

AbstractThe northward migration of the Mendocino triple junction (MTJ) drives a fundamental plate boundary transformation from convergence to translation; producing a series of strike‐slip faults, that become the San Andreas plate boundary. We find that the 3‐D structure of the Pacific plate lithosphere in the vicinity of the MTJ controls the location of San Andreas plate boundary formation. At the time of initiation of the Pacific‐North America plate boundary (∼30 Ma), the sequential interaction with the western margin of North America of the Pioneer Fracture Zone, soon followed by the Mendocino Fracture Zone, led to the capture of a small segment of partially subducted Farallon lithosphere by the Pacific plate, termed the Pioneer Fragment (PF). Since that time, the PF has translated with the Pacific Plate along the western margin of North America. Recently developed, high‐resolution seismic‐tomographic imagery of northern California indicates that (a) the PF is extant, occupying the western half of the slab window, immediately south of the MTJ; (b) the eastern edge of the PF lies beneath the newly forming Maacama fault system, which develops to become the locus for the primary plate boundary structure after approximately 6–10 Ma; and (c) the location of the translating PF adjacent to the asthenosphere of the slab window generates a shear zone within and below the crust that develops into the plate boundary faults. As a result, the San Andreas plate boundary forms interior to the western margin of North America, rather than at its western edge.

Rotation of the Microplates Within the Plate Boundary in Southwestern United States

AbstractI investigate the long‐term, rigid motions of the 20 microplates identified by McCaffrey (2005, https://doi.org/10.1029/2004jb003307) within the Pacific‐North America plate boundary in southwestern United States. Those motions are described by the Euler vectors ( for the ith microplate) given by McCaffrey for each microplate. McCaffrey noticed that the Euler poles for those microplates were aligned along the great circle that connects the geometric center of the microplate distribution with the PACI pole, the pole of rotation of the Pacific Plate (PA) about the North American Plate (NA). To explain that alignment, Thatcher et al. (2016, https://doi.org/10.1002/2015jb0126678.0) proposed replacing each by two, vertical‐axis rotations of the microplate, one describing the trajectory (orbit) of its center of mass (CM) and the other its rotation (spin) about that CM, where . Moreover, they suggested that the orbital motion was being driven by drag from the rotating PA, which suggests that the poles coincide with the PACI pole. Then rotation vectors consistent with the given can be found for 17 of the microplates; the other 3 microplates are apparently affected by Basin‐and‐Range extension as well as PA relative motion. The long‐term motion of each of the 17 microplates then can be described as an orbital rotation about the PACI pole plus spin about the CM of the microplate. The closer the microplate CM is to the PA, the more nearly its orbital rotation rate approaches the rotation rate of the PA about the PACI pole.

Review of Geodetic and Geologic Deformation Models for 2023 U.S. National Seismic Hazard Model

ABSTRACT We review five deformation models generated for the 2023 update to the U.S. National Seismic Hazard Model (NSHM), which provide input fault-slip rates that drive the rate of earthquake moment release. Four of the deformation models use the Global Positioning System-derived surface velocity field and geologic slip-rate data to derive slip-rate estimates (Evans, Pollitz, Shen-Bird, and Zeng), and one model uses geologic data (the “geologic model”). The correlation between the geologic model preferred slip rates and geodetically derived slip rates is high for the Pollitz, Zeng, and Shen-Bird models, and the median of all slip-rate models has correlation coefficient of 0.88. The median geodetic model slip rates are systematically lower than the preferred geologic model rates for faults with slip rates exceeding 10 mm/yr and systematically higher on faults with slip rates less than 0.1 mm/yr. Geodetically derived slip rates tend to the low end of the geologic model range along sections of the San Andreas fault and the Garlock fault, whereas they tend to be higher across north coast California faults. The total on-fault moment rates agree well across models with all rates within 18% of the median. Estimated off-fault strain rate orientations and styles vary considerably across models and off-fault moment rates vary more than on-fault moment rates. Path integrals across the western U.S. accounting for fault-slip rate and off-fault deformation are generally consistent with Pacific-North America plate motion with the median deformation rates recovering about 98% of the plate motion with about 20% of the total plate motion accommodated by off-fault strain rate. The geologic model, which has no off-fault deformation, accounts for about 82% of plate motion with fault slip. Finally, we make a recommendation for relative weighting of the models for the NSHM as well as several recommendations for future NSHM deformation model development.

Late Miocene transition from extension to dextral transtension in the southern Coastal Sonora fault zone, Gulf of California rift, Mexico

The Coastal Sonora fault zone (CSFZ) in northwestern Mexico records the Pacific–North America plate boundary reorganization during the Miocene from subduction to a transform boundary. This study presents the structural architecture of the Sierra El Aguaje-San Carlos-Guaymas region along the southern part of the CSFZ. The study area shows a complex fault pattern divided into four tectonic domains: ENE–WSW left-lateral, normal, and oblique faults in the Sierra El Aguaje and Cerro El Vigía domains and N–S domino-type normal faults in the El Parral and San Carlos domains, which are separated by the NW-striking dextral San Carlos fault. Based on a new fault-slip dataset and K-Ar ages, we propose a structural evolution model in which deformation progressively evolved from extension to dextral transtension. The deformation history involved ∼E–W pure extension associated with N–S normal faults from ∼15.3 Ma to ∼11–10 Ma, followed by transtension and the development of strike-slip faults after ∼11–10 Ma. That change induced the reactivation of preexisting faults and local strain partitioning between normal, oblique, and strike-slip faults. Transtension became strike-slip dominated during the latest Miocene and the Pliocene, accumulating ∼8 km of post-6.3 Ma dextral slip along the San Carlos fault. Paleostress analysis demonstrates that during the extensional phase, σ2 and σ3 were horizontal and oriented ∼N–S and ∼E–W, respectively. The transition to transtension required that σ2 reached the lithostatic stress value, its magnitude approached σ1, and its orientation remained ∼N–S. These conditions implied a horizontal tectonic force directed towards the north began after ∼11–10 Ma in northwestern Mexico. We propose that this new stress regime was related to the end of the Farallon–North America convergence and the inception of the Gulf of California oblique rifting with σ1 oriented N–S.

Feedback between megathrust earthquake cycle and plate convergence

Over million years, convergence between the Nazca and South America tectonic plates results in Andean orogeny. Over decades/centuries, it fuels the earthquake cycle of the Andean megathrust. It is well recognised that, over the geologically-long term of million years, Andean orogeny feeds back onto plate convergence rates, generating temporal changes documented throughout the Neogene. In contrast, no feedback mechanism operated over the geologically-short term by the earthquake cycle is currently contemplated. In fact, it is commonly assumed that the rates of contemporary convergence, which are accurately measured via geodesy, remain steady during the megathrust earthquake cycle. Here we investigate whether the contemporary Nazca/South America plate motion varies over year-/decade-long periods in response to megathrust stress variations associated with the earthquake cycle. We focus on the decade preceding the three largest and most recent {varvec{M_w > 8}} earthquakes (2010 {varvec{M_w = 8.8}} Maule, 2014 {varvec{M_w = 8.1}} Iquique, 2015 {varvec{M_w = 8.3}} Illapel), and find slowdowns of both Nazca and South America whole-plate motions that exceed the impact of data uncertainty or noise. We show that the torque variations required upon Nazca and South America to generate the slowdowns are consistent with that arising from the buildup of interseismic stress preceding the earthquakes.

Evolution of Miocene normal and dextral faulting in the lower Colorado River region near Blythe, California, USA

Abstract The evolution of strain in nascent continental plate boundaries commonly involves distributed deformation and transitions between different styles of deformation as the plate boundary matures. Distributed NW-striking faults, many with km-scale right-lateral separation, are prevalent near Blythe, California, and have been variably interpreted to have accommodated either Middle Miocene NE-SW extension as normal faults or Late Miocene to Pliocene dextral shear as strike-slip faults. However, with poor timing and kinematic constraints, it is unclear how these faults relate to known domains of Neogene deformation and the evolution of the Pacific–NorthAmerica plate boundary. We present kinematic data (n = 642 fault planes, n = 512 slickenlines) that demonstrate that these faults dominantly dip steeply northeast; ~96% of measured faults record normal, dextral, or oblique dextral-normal kinematics that likely reflect a gradational transition between normal and dextral oblique kinematic regimes. We constrain fault timing with 11.7 Ma and 7.0 Ma 40Ar/39Ar dates of rocks cut by faults, and laser ablation–inductively coupled plasma–mass spectrometry U-Pb dating of calcite mineralized during oblique dextral faulting that demonstrates fault slip at ca. 10–7 Ma and perhaps as late as ca. 4 Ma. This Late Miocene dextral oblique faulting is best compatible with a documented regional transition from Early to Middle Miocene NE-directed extension during detachment fault slip to subsequent NW-directed dextral shear. We estimate 11–38 km of cumulative dextral slip occurred across a 50-km-wide zone from the Palen to Riverside mountains, including up to 20 km of newly documented dextral shear that may partly alleviate the regional discrepancy of cumulative dextral shear along this part of the Late Miocene Pacific–North America plate boundary.

Evidence of South American lithosphere mantle beneath the Chile mid-ocean ridge

Numerous geochemical studies on mid-ocean ridge basalts have established the presence of both compositional and lithological heterogeneities in the upper mantle. These studies have successfully constrained the composition and age of the chemical heterogeneities within the Earth's upper mantle. However, the origin of those heterogeneities remains highly debated. The Chile Mid-Ocean Ridge in the southeast Pacific Ocean, located far away from any hotspot, is colliding and subducting under the South America plate promoting the formation of a slab window. Comprehensive geochemical data (major, trace, volatile element contents and Sr, Nd, Hf and Pb isotope ratios) on Chile Ridge submarine glasses collected over the 1000 km ridge length demonstrate significant mantle compositional variability. Four main mantle components have been recognized: the typical Pacific MORB mantle, an enriched mantle (e.g., EM-1), a Subduction Modified mantle, and an anciently depleted mantle with unusually high Hf isotope ratios. Surprisingly, despite the large compositional variability, all glasses – including those with a subduction signature – have volatile-refractory element ratios within the range of the Pacific normal MORB. We propose that the Patagonia sub-continental lithospheric mantle, variably metasomatized since the Early Paleozoic, is eroded and incorporated into an asthenosphere with a south Atlantic MORB mantle composition that is flowing westward across the slab window from South America to the Chile Ridge. This scenario contrasts with a well-accepted geodynamic model that predicts the opposite direction of mantle flow. We speculate that the westward mantle flow across the slab window might be a counter-flow response to the opening of the Drake passage and the eastward movement of Pacific mantle since ∼30 Ma ago.

Seismicity at the intersection of the Coast Shear Zone and Anahim Volcanic Belt near Bella Coola, British Columbia, Canada

In the Coast Mountains of western British Columbia, an anomalous seismicity concentration exists near the intersection of the Coast Shear Zone, a major northwest–southeast trending Eocene-age shear zone that accommodated deformation between the Pacific and North America plates, with the Anahim Volcanic Belt, an east-northeast–west-northwest trending zone of volcanic features that decrease in age to the east. To better characterize seismicity in the Coast Mountains, we augment the existing Natural Resources Canada seismicity catalogue by applying an automatic detection and location algorithm to both permanent Canadian National Seismic Network stations and temporary stations from the 2005–2006 BATHOLITHS deployment, resulting in 837 relocated events with at least three paired P- and S-phase picks. Double-difference relocation reveals several small-scale linear strands subparallel to the Coast Shear Zone and within the Anahim Volcanic Belt and three clusters of events striking at a high angle to the Coast Shear Zone that occurred as swarms in 2015 and 2017. First-motion focal mechanisms exhibit extensional and strike-slip faulting. Our observations indicate that most of these events are not associated with surficial processes such as landslides, but rather, we hypothesize that the interaction of the Anahim Volcanic Belt and Coast Shear Zone has weakened the lithosphere in this region, leading to current-day strain localization and high heat flow that manifest seismicity, including swarm-like activity.

Active faulting in the Beagle Channel (Tierra del Fuego)

AbstractIn Tierra del Fuego, the Magallanes‐Fagnano Fault System (MFFS) accommodates a significant portion of the relative motion between the South America and Scotia plates. However, it remains unclear whether some of the deformation is partitioned southwards, along the Beagle Channel Fault System (BCFS). In this paper, high‐resolution seismic reflection profiles were used to identify fault‐related ruptures in the submerged Quaternary sediments of the Beagle Channel. Some faults reach the seafloor, affecting marine sediments, indicating they are Holocene in age. The correlation with outcrop data and lineaments mapped onshore suggests the post‐glacial reactivation of two structures: the E‐W striking BCFS and the NW‐SE‐trending Lapataia Fault Zone (LFZ). Whereas the BCFS displays along‐strike variation in throw, the LFZ shows significant normal displacements. These results imply that deformation occurs in a wider and more complex manner than previously thought and highlight the need for a thorough hazard assessment of the area.

Cenozoic slip along the southern Sierra Nevada normal fault, California (USA): A long-lived stable western boundary of the Basin and Range

AbstractThe uplift history of the Sierra Nevada, California, is a topic of long-standing disagreement with much of it centered on the timing and nature of slip along the range-bounding normal fault along the east flank of the southern Sierra Nevada. The history of normal fault slip is important for characterizing the uplift history of the Sierra Nevada, as well as for characterizing the geologic and geodynamic factors that drove, and continue to drive, normal faulting. To address these issues, we completed new structural studies and extensive apatite (U-Th)/He (AHe) thermochronometry on samples collected from three vertical transects in the footwall to the east-dipping southern Sierra Nevada normal fault (SNNF). Our structural studies on bedrock fault planes show that the SNNF is a steeply (~70°) east-dipping normal fault. The new AHe data reveal two elevation-invariant AHe age arrays, indicative of two distinct periods of cooling and exhumation, which we interpret as initiation of normal faulting along the SNNF at ca. 28–27 Ma with a second phase of normal faulting at ca. 17–13 Ma. We argue that beginning in the late Oligocene, the SNNF marked the now long-standing stable western limit, or breakaway zone, of the Basin and Range. Slip along SNNF, and the associated unloading of the footwall, likely resulted in two periods of uplift of Sierra Nevada during the late Cenozoic. Trench retreat, driven by westward motion of the North American plate, along the Farallon–North American subduction zone boundary, as well as the gravitationally unstable northern and southern Basin and Range pushing on the cold Sierra Nevada, likely drove the late Oligoceneaged normal slip along the SNNF and the similar-aged but generally local and minor extension within the Basin and Range. We posit that the thick proto–Basin and Range lithosphere was primed for late Oligocene extension by replacement of the steepening Farallon slab with hot and buoyant asthenosphere. While steepening of the Farallon slab had not yet reached the southern Sierra Nevada by late Oligocene time, we speculate that late Oligocene slip along the SNNF reactivated a late Cretaceous dextral shear zone as the Sierra Nevada block was pulled and pushed westward in response to trench retreat and gravitational potential energy. The dominant middle Miocene normal faultslip history along the SNNF is contemporaneous with high-magnitude slip recorded along range-bounding normal faults across the Basin and Range, including the east-adjacent Inyo and White mountains, indicating that this period of extension was a major regional tectonic event. We infer that a combination of slabdriven trench retreat along the Juan de Fuca–North America subduction zone boundary and clockwise rotation of the southern ancestral Cascade Range superimposed on continental lithosphere preconditioned for extension drove this episode of middle Miocene normal slip along the SNNF and extension to the east across the Basin and Range. Transtensional plate motion along the Pacific–North America plate boundary, and likely a growing slab window, continued to drive extension along the SNNF and the western Basin and Range, but not until ca. 11 Ma when the Mendocino triple junction reached the latitude of our northernmost (U-Th)/He transect.

Recent Euler pole parameters and relative velocities of the Nubia–Eurasia and Nubia–South America plates estimated using GPS technique

Recent Euler pole parameters and relative velocities of the Nubia–Eurasia and Nubia–South America plates estimated using GPS technique

Rapid absolute plate motion changes inferred from high-resolution relative spreading reconstructions: A case study focusing on the South America plate and its Atlantic/Pacific neighbors

The reconstruction of past plate motions relative to a deemed–to–be–fixed hotspot reference frame relies on the sparse occurrence of intraplate volcanism. Consequently, this absolute reference frame often features a temporal resolution that exceeds the rapid kinematic changes observed in plate–to–plate spreading reconstructions, changes recently shown to occur within less than 5 Ma. In this work we put forward an alternative method based on the study of high–resolution relative plate motion data sets. By studying time periods featuring a relatively high probability of plate motion change across multiple spreading ridges, we are able to identify and quantify (likely) changes in absolute plate motion. Specifically, we implement such approach and provide well–defined estimates for the absolute plate motion changes of South America and neighboring plates Nubia, Antarctica, Somalia, North America and Pacific. We find that kinematic changes for all these plates occur between 9 and 5 Ma. For South America, we identify a change also between 14 and 10 Ma. Lastly, we estimate the torque–variations required upon these plates to generate the inferred kinematic changes, which we find to be between ∼5⋅1023 and ∼20⋅1024 N ⋅ m.

Double‐Difference Adjoint Tomography of the Crust and Uppermost Mantle Beneath Alaska

AbstractWe perform adjoint waveform tomography to reveal the P‐wave velocity structure of the crust and uppermost mantle in Alaska by using common‐source double‐difference traveltime data. Our underlying forward modeling tool is a 3D seismic‐wave solver called SPECFEM3D_GLOBE. We select ∼13,000 high‐quality P‐wave arrivals from 147 earthquakes recorded by more than 600 stations. The waveforms are filtered between 15 and 8 s and used to generate about 100,000 differential traveltimes via the cross‐correlation technique. We invert these common‐source differential traveltimes for a P‐wave velocity model down to 150 km depth in Alaska. In the upper mantle, a high‐velocity zone is imaged along the Aleutian volcanic arc, which represents the Pacific plate subducting beneath the North America plate. On the eastern edge of the subducting Pacific plate, the imaged high‐velocity anomaly extends distinctly beyond the Wadati–Benioff zone, indicating an aseismic slab edge. We observe an absence of a low‐velocity mantle wedge beneath the Denali Volcanic Gap (DVG) in our tomographic model. This finding suggests that minimal melt accumulation exists beneath the DVG, which may explain the cessation of magmatism there. Our model also reveals a high‐velocity anomaly near the Wrangell Volcanic Field (WVF), suggesting the possible existence of the Wrangell slab or the high‐velocity slab edge of the Yakutat slab. A potential slab gap shown as a low‐velocity body is detected at 95–125 km depth near the WVF, which could act as a channel to transport mantle materials to feed the cluster of volcanoes in the WVF.

Estimating Coseismic Deformation of Southwestern Puerto Rico from the 7 January 2020 Mw 6.4 Earthquake: Constraints from Campaign and Continuous GPS

ABSTRACT The Puerto Rico–Virgin Islands (PRVI) block lies within the Northern Caribbean Plate Boundary Zone—a zone accommodating stresses between the larger North America and Caribbean plates. Data from Global Positioning System (GPS) sites throughout the PRVI block have been used to confirm the existence of a distinct microblock in the southwest. It is no coincidence that this portion of the PRVI block is the epicentral region of the 7 January 2020 Mw 6.4 earthquake and the ensuing seismic sequence. Prior to the mainshock, the southwestern Puerto Rico (SWPR) region exhibited most of the onland seismic activity. The 2020–2021 SWPR earthquake seismic sequence has been characterized by having an atypical aftershock decay distribution occurring along multiple faults. As a result, fault parameters of the 7 January 2020 mainshock have been poorly defined by conventional seismic methods. Here, we present results from campaign and continuous GPS sites in SWPR, and compare GPS-derived displacements to those computed from the U.S. Geological Survey National Earthquake Information Center (NEIC) focal mechanism. We conclude that irrespective of which nodal plane is used, the observed coseismic displacements from GPS differ from those predicted using a simple elastic model and the NEIC focal mechanism. We infer based on these observations that the complex mainshock rupture resulted in a suboptimal double-couple solution.

Neotectonic faults in the Southern Chile intra-arc (38°S–40.5°S): Insights about their seismic potential and the link with the megathrust earthquake cycle

Megathrust earthquakes are a primary geological hazard in subduction zones. In oblique subduction margins, however, seismic threat must be also considered concerning crustal strike-slip faults driving the parallel-to-the-trench component of the convergent vector. These faults have proven their capacity to produce moderate-to-large shallow earthquakes, causing severe damage for surrounding areas. The Southern Chile Subduction Zone (SCSZ) is characterized by the oblique convergence between the Nazca and the South America plates. Between 37°S and 46°S, slip partitioning is significantly led by intra-arc faults like the Liquiñe-Ofqui Fault System (LOFS) and subsidiary NW to NE striking structures. Despite it has been demonstrated that the behavior of these faults is seismogenic, the inland evidence of Holocene deformation along them is scarce. This has concealed to accurately define individual active faults, discussing their seismic potential, and addressing their link with the megathrust earthquake cycle. In this contribution, we first present field evidence for Holocene deformation on intra-arc faults in the area. Based on this, we define 5 active faults and discuss their seismic capacity. To assess the link with the megathrust earthquake cycle, we calculate the Coulomb stress changes induced on these faults by the Mw 9.5 Valdivia Earthquake slip distribution and an interseismic period of 300 years. We conclude that 1) the identified neotectonic faults have the potential for producing moderate-to-large earthquakes (Mw 5.5 to 6.8) and 2) their occurrence would be enhanced by the coseismic and the interseismic stages of the subduction cycle. With this, we propose that the intra-arc faults addressed in this paper, and certainly others in the SCSZ, must be considered as significant sources of seismic hazard.